Electrophotographic machines, such as, for example, copiers and printers, produce images by forming a latent image charge pattern on a photoconductive surface. The photoconductive surface carries the latent image through a developing station wherein pigmented toner particles are drawn by electrostatic attraction onto the latent image charge pattern on the photoconductive surface. An electric field is applied to transfer the image from the photoconductive surface onto either an intermediate transfer member or an image substrate, such as, for example, a piece of paper. Thereafter, the image is fixed, such as, for example, by fusing, to the image substrate. The fusing process applies heat and pressure to the image substrate, and is typically carried out by a fusing nip formed between a heated fusing roller and an opposing pressure roller. The fusing roller may be internally or externally heated, or some combination thereof.
The heat applied to an internally heated fusing roller must diffuse through the roller and its outer surface. Since heat is applied directly to the outer surface of an externally heated roller, the need for heat to diffuse through the roller and its outer surface is eliminated. Externally heated fusing rollers, therefore, have a much faster thermal response than internally heated fusing rollers. Accordingly, an externally heated fusing roller can typically be heated to a given operating temperature more rapidly and can employ a thicker outer cushioning layer to improve the efficiency and reliability with which paper releases from the fusing roller.
However, externally heated fusing rollers are disadvantageous in that the roller itself does not act as a heat reservoir to the same extent that internally-heated fusing rollers do. Therefore, at least during the first few fusing operations, an undesirable and sharp reduction in the temperature of the fusing roller surface may occur due to the significant amount of heat that is transferred from the fusing roller surface to the image substrate. This short-term reduction in the surface temperature of the fusing roller will be especially pronounced during the first few fusing operations, i.e., the fusing operations that occur during the delay from the time at which the reduction in the fusing roller surface temperature is first sensed to the time at which the fusing roller surface is returned to nominal temperature. This short-term reduction in fusing roller surface temperature is undesirable in that one or more image substrates may be exposed to fusing process parameters that are less than optimal/nominal.
Therefore, what is needed in the art is an improved method for controlling the surface temperature of an externally heated fusing roller.
Furthermore, what is needed in the art is a method that reduces the pronounced reduction in the temperature of the surface of an externally heated fusing roller that may occur during the initial operation thereof.
The fusing roller surface temperature is also subjected to longer-term temperature variation due to various factors, including electrical noise, variations in image substrate or media thickness and/or weight, and diffusion of heat from the internal lamp to the fusing roller surface. Conventionally, such long-term variation in fusing roller surface temperature is compensated for by a control method, such as a proportional integration derivative method that adjust the power applied to the heating rollers and/or the force with which the heating rollers engage the fusing roller. However, such conventional control methods may result in undesirable operating conditions, such as, for example, wherein the heating roller engages the fusing roller with zero engagement force or maximum engagement force.
Thus, what is needed in the art is an improved method of controlling fusing roller surface temperature.